This is a specialist type of imaging. A long, thin probe is inserted into the anus and rectum. An ultrasound scanner is built into this probe and circumferential images can be taken of the lower most part of the bowel and the anus. This is a very useful addition to a clinical examination.  
Endoanal ultrasound is used to look for:

1) Fistula in ano.
A scan can be very useful in showing the anatomy and direction of the fistula.
This aids the subsequent surgery.  

2) Infection.
A scan is very sensitive to looking for pockets of infection or abscesses around the anus.

3) Anatomy.
Some surgery involves operating on or very close to, the sphincter muscle (the muscle which controls continence). The endoanal ultrasound is very useful in looking at the length and thickness of this sphincter in order to help guide the surgeon through an operation. It can also be used in this role to look for any hidden damage to the muscle, for example, following tears during childbirth.